I was moving along nicely at 110 mph or so driving from Dresden back to Frankfurt and a C5 RS6 Avant, Mugello Blue, came up on my tail, backed off and checked me out, I checked him out, and I noticed an "R" under the RS6. He accelerated to around 140 mph and so did I and I did not feel comfortable going faster because of the traffic and he stomped on it and flew away. Very fast car, then slowed backed down and we both cruised at about 100 mph or so for many miles, given the traffic. It was fun, but what is the "R"?